1. Field of the Invention
This invention relates generally to a process for forming a high-quality dielectric layer for a semiconductor substrate. More particularly, it relates to a better controlled process utilizing atomic nitrogen and nitric oxide to form a more reliable and predictable dielectric layer.
2. Description of Related Art
In order to build an integrated circuit, many active devices need to be fabricated on a single surface. The current practice in many areas of semiconductor manufacturing is to use thin film fabrication techniques. A large variety of materials can be deposited using thin films, including metals, semiconductors, insulators and the like. The composition and uniformity of these thin layers must be strictly controlled to facilitate other necessary fabrication techniques, such as the etching of submicron features onto the circuit or component. The dielectric material is a pivotal component in the manufacture and performance of integrated circuits that must respond to the demand for decreasing size of device geometries and simultaneous increases in packing density. The characteristics of the dielectric exert a large influence over the long-term reliability of every device in which a dielectric is present.
Metal oxide semiconductor (MOS) device technology increasingly requires ultrathin dielectric layers. Ultrathin dielectric layers must meet existing demands for performance and integrity in the face of the added constraint of an ultrathin layer.
It has been recognized that silicon dioxide (SiO.sub.2) cannot adequately meet the rigorous requirements for a dielectric as technology improvements require thinner and ultrathin dielectric layers.
In the search for acceptable dielectric materials to meet the more stringent requirements, several alternatives have been pursued. Two widely used alternatives are silicon nitride (Si.sub.3 N.sub.4) and oxynitrides.
Silicon nitride (Si.sub.3 N.sub.4) can be deposited using a number of techniques. Those techniques fall into two general categories: low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD). The LPCVD methods use anhydrous ammonia (NH.sub.3) as the nitrogen source, and, typically, silicon hydride (SiH.sub.4) or SiCl.sub.2 H.sub.2 to provide the silicon for the deposition. The temperature ranges associated with the LPCVD of silicon nitride generally are between 600.degree. and 900.degree. C.
The PECVD methods of depositing silicon nitride typically use SiH.sub.4 as the source of silicon and either NH.sub.3 or N.sub.2 as a nitrogen source, at lower temperatures than the LPCVD techniques. PECVD temperatures for silicon nitride generally are in the range of 200.degree. to 400.degree. C. Within these broad categories of techniques, refinements in the deposition process have been introduced. For example, U.S. Pat. No. 5,298,629 discloses a method of PECVD of silicon nitride using nitrogen (N.sub.2) or ammonia (NH.sub.3) in a plasma chamber containing a silicon body and following this step with an ammonia treatment in a non-plasma atmosphere.
Currently, the Si.sub.3 N.sub.4 layers are reoxidized after deposition to improve their dielectric qualities. For example, the reoxidation operates to reduce defects such as pinholes and leakage current. The reoxidation of the Si.sub.3 N.sub.4 layers typically is accomplished by high-temperature treatment in steam or an oxygen (O.sub.2) ambient environment.
With current fabrication techniques the Si.sub.3 N.sub.4 films cannot be reduced below approximately 65 .ANG. because a Si.sub.3 N.sub.4 layer thinner than this cannot withstand the subsequent oxidation step used to improve the dielectric qualities of the Si.sub.3 N.sub.4 layer. But, as discussed above, thinner layers are increasingly required to accommodate the smaller device geometries and increased packing densities.
Silicon oxynitrides are another class of dielectric material currently in wide use. While similar to reoxidized Si.sub.3 N.sub.4, they also represent an alternative manufacturing choice for dielectric layer formation. Several methods are used to form oxynitride layers. In one method, a silicon substrate is first oxidized to silicon dioxide and then overlaid with a polysilicon layer that is implanted with nitrogen, followed by an annealing step. In another method, a silicon substrate is treated using a rapid thermal annealing step in a nitrous oxide ambient environment. Silicon oxynitride layers are also produced by deposition processes using silicon hydride (SiH.sub.4), ammonia (NH.sub.3) and either nitric oxide (NO) or nitrous oxide(N.sub.2 O) to produce films having specific compositions. The films using N.sub.2 O have shown good electrical characteristics, but have inadequate nitrogen incorporation to prevent boron penetration and incorporation. The use of NO results in more nitrogen incorporation in the layer, but the growth of the oxynitride layer is undesirably slow.
The trend towards smaller geometries and more densely packed devices in IC fabrication requires attendant accommodations in dielectric formation methods. Dielectric fabrication needs to be reworked to oxidation methods that thinner Si.sub.3 N.sub.4 layers can tolerate, as well as more controlled and responsive oxynitride growth methods.